This invention is related, in general, to low noise amplifiers and, more specifically, to low noise amplifiers having an amplifier mode and a bypass mode.
It is common for a transmitted Radio Frequency (RF) signal received through an antenna to be amplified prior to being passed to a mixer for frequency conversion. The mixer modulates the received RF signal with a local oscillator signal and generates an Intermediate Frequency (IF) output signal having a frequency that is the difference of the received RF signal frequency and the local oscillator signal frequency. When the signal strength of the received RF signal is low, an amplifier provides signal gain prior to the RF signal being mixed. On the other hand, when the signal strength of the received RF signal is high, the RF signal is not amplified prior to being mixed to prevent overdriving the mixer.
Prior art amplifiers that switch operating modes between providing signal gain and not supplying signal gain, change the input and output impedances of the amplifier based on the mode of operation. Any impedance change at either the input or output of the amplifier causes undesired changes to the filters preceding and following the low noise amplifier. The filter before the low noise amplifier is needed to separate the receive signal from the transmitted signal. The filter following the low noise amplifier is needed to reject the image signal and is well known to those versed in the art. Both of these filters require a fixed input and output impedance, which is usually 50 ohms.
By now it should be appreciated that a low noise amplifier is needed having both an amplifying mode and a bypass mode and an input and output impedance that is substantially constant when the amplifier is switched between the modes of operation.